The present invention relates to a friction engaging device, such as, for example, a braking disc for a friction brake or friction clutch.
More particularly the present invention relates to a high performance friction engaging device, such as, for example, a braking disc for use in a guided trackway vehicle.
The choice of materials used for making friction engaging devices is determined by the conditions under which a vehicle will operate and other surrounding circumstances.
By way of example, trains or their rolling stock conventionally have disc brakes which utilise steel or cast iron braking discs. Using such steel braking discs, the current maximum performance for a conventional train brake is that proposed for a new high speed passenger train where the maximum energy per steel braking disc in one stop is 22 MJ, with an average deceleration calculated from 350 km/h to stationary of about 0.7 msxe2x88x922. 
For trains or their rolling stock, which can have multiple brakes comprising at least four axle mounted braking discs per axle, a low margin between recovered fares and operating costs means the market price of a braking disc is relatively low and the life time requirement for the braking disc is high.
Also at around 100 kg for a single braking disc, each axle may have 400 kg of braking discs. Thus a greater number of braking discs adds to the weight of an axle and the greater the weight of the axle the greater the resulting track damage. Furthermore, since a maximum of four steel braking discs, due to their bulk, can be mounted on an axle, it is not very easy to increase the vehicle speeds much beyond 350 km/h using steel braking discs.
It would therefore be a clear advantage to reduce the number and/or weight and/or the bulk of these braking discs and/or improve their performance.
For trains, emergency stops at maximum rating have to be achievable with subsequent normal braking of the vehicle continuing after without any re-fitting of service parts. Consequently, good wear characteristics are very important.
In contrast, the braking discs for an aircraft""s brakes, are in part determined by the type of flights they undergo. A jumbo jet may have as many as 9xc3x9716 clutch braking discs which are made of carbon-carbon fibre ie. a carbon fibre reinforced carbon composite. The current maximum performance for such a carbon-carbon fibre clutch brake is 71 MJ with an average deceleration from about 290-320 km/h to zero of about 4.2 msxe2x88x922, this being the performance required to cater with an aborted take-off. In contrast to a train, where subsequent normal braking with the same braking discs is required, the whole of the undercarriage of the aircraft, including the braking discs, has to be replaced after such an aborted take-off. Consequently, with an aircraft it is less important that the braking discs wear characteristics are poorer than those provided by steel braking-discs.
In aircraft where journey times are short and stops are frequent eg. a domestic shuttle operation, steel braking discs may be preferred as steel, unlike carbon-carbon fibre, does not give rise to judder and gives better wear characteristics thus making steel braking discs more economical despite the need for increased fuel load, due to heavier brakes.
The decreased weight of carbon-carbon fibre braking discs and their ability to operate at relatively high temperatures makes them attractive for use on other vehicles. So why not use carbon-carbon fibre for the braking discs to be used on train brakes? This has indeed been tried with little success due to a lack of frictional stability and a poor wear life under operational conditions.
To explain, the dry friction behaviour of carbon-carbon fibre is complex. It exhibits unstable friction at low temperatures (less than 300xc2x0 C.) and the wet friction can be very low (xcexc=0.05) at ambient temperatures. Wear behaviour is poor at low speeds and low temperatures (called snub stops) due to high friction and also at high temperatures (greater than 600xc2x0 C.) due to oxidation. The above currently makes carbon-carbon fibre braking discs unsuitable for use on trains. In aircraft snub stops cause a lot of judder, primarily experienced when an aircraft is taxiing. The wear caused by such snub stops is often large compared to wear caused by landings at much higher energy levels. Overcoming this problem would therefore be of benefit to the aircraft industry. Also very high energy stops cause excessive wear owing to oxidation of the carbon which begins at 500xc2x0 C. and rapidly accelerates as the temperature increases. Despite these observations carbon-carbon fibre is a useful material for braking discs where the majority of braking is done with the braking discs at middle range temperatures (say, 250xc2x0 C. to 600xc2x0 C.), which is the case for aircraft landings and Formula One racing cars. Also, these vehicles have disc brakes which are enclosed so that the braking discs do not get intermittently wet in service.
All of these factors means that generally different materials are preferred for the braking discs of trains and aircraft due to the different operational conditions they experience.
It is an aim of the present invention to develop a friction engaging device which overcomes at least some of the abovementioned problems and/or disadvantages of the prior art devices.
According to a first aspect of the present invention there is provided a friction engaging device in the form of a carbon-ceramic composite comprising a carbon fibre network and a filler comprising silicon carbide.
Such a composition can be made utilizing a number of known processes.
In one embodiment the carbon-ceramic composite comprises, by volume, 10 to 60% of a carbon fibre network and up to 90% by volume of a filler comprising silicon carbide.
The filler may comprise a volume of air (porosity).
According to a second aspect of the present invention there is provided a friction engaging device in the form of a carbon-ceramic composite consisting a carbon fibre network and a filler.
Preferably the filler consists essentially of silicon carbide. Alternatively the filler may consist of silicon carbide, silicon oxide, silicon and free carbon.
In another embodiment, the carbon-ceramic composite comprises, by volume, 10 to 60% of a carbon fibre network and 40 to 90% of a filler more preferably 30% of a carbon fibre network and 70% of a filler.
The overall composition, by end weight percent, comprises:
The preferred ranges and actual weights will depend upon the carbon fibre content and the degree of impregnation. The porosity has no effect on the weight.
For a material produced from a starting material with a carbon fibre content of 30% by weight of the carbon-carbon composite, the carbon-ceramic composite will comprise (end weight percent):
For a material produced from a starting material, with a carbon fibre content of 10% by weight of the carbon-carbon composite after impregnation the carbon-ceramic composite will comprise (end weight percent);
and, for a material produced from a starting material with a carbon fibre content of 60% by weight of carbon-carbon composite after impregnation the carbon-ceramic composite will comprise (end weight percent).
Of course, the relative end weight percentages of free carbon to silicon carbide in these embodiments could be decreased if more silicon were to be introduced into the system. Theoretically, the amount of free carbon could be reduced to zero (as per the first embodiment) in which case the amount of silicon carbide would well exceed the maximum figure given in this embodiment.
The carbon fibre network provides the friction engaging device with its good tensile properties and strength at high performance levels whilst the silicon carbide in the filler, provides the composite with its good wear, oxidation resistance and thermal properties. Such a device overcomes many of the problems associated with the prior art devices.
According to a third aspect of the present invention there is provided a friction engaging device in the form of a carbon-ceramic composite, preparable by substantially filling, by impregnation, a free carbon containing volume, defined as the volume between respective fibres of a carbon fibre network, with silicon under conditions whereby all or substantially all of the carbon fibre network remains as carbon fibre, and a proportion of the free carbon present in the volume is converted within the volume to silicon carbide by reaction with the silicon.
By impregnation is meant the ability to add material by liquid and/or gaseous infiltration and/or by chemical reaction eg. diffusion without a significant increase in physical size.
According to a further aspect of the present invention there is provided a method of producing a friction engaging device of the invention in which a carbon-carbon composite comprising 10 to 60% by weight of a carbon fibre network and from 40 to 90% by weight free carbon is impregnated with silicon under conditions such that substantially all of the carbon fibre network remains as carbon fibre, and a proportion of the free carbon present in the volume is converted within the volume to silicon carbide by reaction with the silicon.
Preferably about 10 to 35%, by volume, of the free carbon is converted to silicon carbide, more preferably still about 20% for a composite made by impregnating a 30% by volume carbon-carbon composite.
Preferably the friction engaging device is produced from a carbon-carbon composite with a density of 1.4 to 1.8 g/cm3.
Preferably the carbon-carbon composite has an open porosity of 10-30%, more preferably still 15%.
Preferably, the carbon-carbon composite has a thermal conductivity in an axial and radial direction of at least 12 W/mK.
The carbon-carbon composite may comprise from 10 to 60% by weight of a carbon fibre network and from 40 to 900% by weight free carbon, any air (porosity) present in the structure having no effect on the weight.
If a braking disc according to the invention is made using the REFEL process, the carbon-carbon composite which is subjected to impregnation with silicon may comprise from 10 to 60% by weight of a carbon fibre network and from 90 to 40% by weight of free carbon.
During processing the volume which previously comprised free carbon and air is impregnated with silicon and the air content (porosity) thereby reduced, typically to about 5%. Since up to about 163%, and at least about 82% by weight silicon may be introduced into the volume by impregnating the open space (porosity) with silicon by the REFEL process the silicon can be reacted with a proportion of the free carbon to produce silicon carbide.
In practice about 430% to 78% by weight of the free carbon remains as free carbon, the remaining about 22 to 57% by weight being converted to silicon carbide for a composite made by impregnating a 30% by volume carbon fibre carbon-carbon composite. However not all the silicon introduced is converted into silicon carbide, some remains as silicon and some is converted to a silicon oxide.
The REFEL processing conditions are substantially as described in U.K. Patent Nos:1,437178 and 1,596303. However, it has been found that a silicon carbide seed is not essential to initiate the process to form the device of the present invention.
In one embodiment of the present invention a friction engaging device is produced as follows:
A carbon-carbon composite, such as that used in the manufacture of braking discs used in the aircraft and Formula One Racing Car industries, with a density between 1:4 and 1:8 g/cm3, an open porosity between 10% and 30% and a thermal conductivity of at least 12 W/mK in the axial and in any radial directions, was machined to a desired shape by, for example, milling, grinding and turning. The carbon-carbon composite was thoroughly dried and silicon introduced substantially as per the REFEL process, the shaped carbon-carbon composite friction engaging device or a part therefore being placed in a refractory crucible with elemental silicon and subjected to heat (over 1600xc2x0 C.) in an inert atmosphere (i.e. Argon) or under vacuum. By melting elemental silicon in the presence of the shaped carbon-carbon composite, the liquid, and vapour silicon, penetrates the volume via the open pore structure between the carbon fibre network, and a proportion of the free carbon, but unexpectedly not the carbon fibres, is converted into silicon carbide.